This invention relates to rubber screens for use in a vibratory screening apparatus for ore concentrates in mines, blast furnaces, etc., which effectively prevent the blockage or clogging of the screen openings.
Conventional screens for iron ores, cokes, broken stones, and the like include knitted steel wire meshes, punched iron sheets, rubber screens, wedge wires, and polyurethane screens. These conventional screens have not proved entirely satisfactory, however, for one or more reasons described below with reference to FIGS. 1 to 4, in which FIG. 1 is a plan view of a conventional rubber screen with a feed particle thereon; FIG. 2 is a plan view showing the feed particle in FIG. 1 wedged into a screen opening; and FIGS. 3 and 4 are front elevations showing the particle wedged into opening, and about to fall through, respectively.
Woven metallic wire meshes and punched iron sheets have poor abrasion resistance, are frequently clogged which reduces their screening efficiency, and are very noisy in operation. Of the synthetic rubber and polyurethane screens, those of the type shown in FIGS. 1 to 4 include coreless rope members 2 arranged parallel to the feed direction (shown by the arrow), and cored rope members 1 arranged transverse to the particle flow. The rope members 1 have embedded tensile cores 3 with a low elongation coefficient, and the resulting screen undergoes comparatively little clogging. The flexible coreless rope members 2 have a high elongation coefficient, however, whereby particles S larger than the screen mesh frequently become wedged into a opening and gradually work through, as shown in FIG. 4. Thus, the properly sorted undersize particles that have fallen through the screen often contain a number of larger particles of undesired size. Further, in order to increase the abrasion resistance of the screen, the diameter of the rope members must be increased, which results in a decreased ratio of screen openings and a correspondingly reduced screening efficiency.
It is generally necessary to reduce the screen mounting tension in order to prevent clogging. In screens of the aforementioned type, however, when the mounting tension is low, the screen flutters and incessantly collides with the support frame mounted on the back of the screen. This causes screen or tensile member breakage, which markedly shortens the service life of the screen.
Present day rubber screens also include strong tensile member cores extending in both the transverse and parallel directions and the sorted particle diameter is more stable with such screens. Since low-elongation, high-modulus twisted wires are used as the cores, however, the tensile members tend to hold wedged feed particles firmly in place in the screen openings, which causes substantial blockage or clogging. Specifically, when the mesh size is less than 15 mm in the lateral and the longitudinal directions, respectively, a low tension mounting must be used for the screen in order to prevent such clogging, and as mentioned above such low tension causes undesirable collisions between the screen and the support frame.